Integrated circuits ("ICs" or "chips") are widely used in complex electrical an electronic applications where reliability is essential. Testing the chips while they are attached to or soldered in a circuit board is often necessary. However, due to the small, compact nature of these chips, the testing of ICs with dozens of pins is often a tedious process.
Two types of passive voltage probes, high-resistance probes and low-resistance probes, presently exist in the prior art to test integrated circuits.
The high-resistance probes are limited to a 300-400 MHz IC signal bandwidth, due to high impedances, and are designed to interface with oscilloscopes or other measuring systems typically having 1M.OMEGA. inputs. These high impedance probes generally have an input capacitance of 10 pf. The low-resistance and low-capacitance probes can sense signals with bandwidths exceeding several GHz and are designed to interface with 50.OMEGA. inputs. In such a probe, the acquired signal travels in a controlled-impedance environment, thereby preserving the high-frequency characteristics of the signal.
Achieving sensitivity to high-frequency signals with both types of prior art probes requires a test measurement to be made with the shortest possible distance between the probe tip and the signal, and also a very short distance between the probe tip and the probe ground lead. Due to the nature of the probe construction, a relatively long ground lead is usually present, unless the probe is inserted into a specially designed probe socket, at a fixed point on the circuit board. Thus, it is seldom possible to use a probe socket unless a specialized test point was included in the circuit board design. Also, to test a plurality of IC signals, the probe must be separately attached to each pin under test, a very slow process.
U.S. Pat. No. 4,749,362 discloses a device that clips or clamps onto a dual in-line package (DIP) IC. The clip has contacts which are pressed against the IC pins and are coupled to extended pins on top of the DIP clip. A probe is attached to the clip pins for monitoring signals of the IC. The ground lead of the probe is connected to the ground pin of the IC, usually (but not always) located the left lower corner pin of the IC. Although the clip conveniently attaches to a DIP IC, the arrangement is not suitable for high-frequency measurement because the distance from an IC pin to the probe tip is relatively long. The ground return path is also very long. As a result, to the measurement system the signal path looks inductive, and the measured wave forms are distorted. In extreme cases, high frequency, narrow pulse signals are lost altogether. In many high-speed digital circuits, a 10 pf circuit loading together with the inductive component of the probe can produce very undesirable results, including overshoot, ringing and oscillation. In the case of very narrow pulses, a 10 pf loading can make the signal disappear all together. In contrast, loading by a low-resistance and low-capacitance probe in the 500.OMEGA. to 5K.OMEGA. range with less than 1 pf capacitance often produces negligible effects, so low-resistance low-capacitance probes are highly desirable.
U.S. Pat. No. 4,419,626 provides a means capacitively offsetting the distortion, but it uses a complex circuit requiring inductors and capacitors.
Low-resistance probes can be connected to 50.OMEGA. transmission line systems, enabling the acquired signals to be easily processed through high-speed scanners, attenuators, and digitizing systems for automatic measurements. On the other hand, it is virtually impossible to multiplex high-frequency signals through high-resistance circuits. That is, it is very difficult to build a stand alone high-frequency and high-impedance attenuator. A high-input impedance amplifier is normally required to capture the signal from the attenuator output.
Thus, those of ordinary skill in the art would find desirable a controlled-impedance integrated circuit test system for testing integrated circuits installed in a circuit board, providing very short lead length from the test probe system to the device under test, enabling high-frequency signal measurement of a plurality of signals.
Such persons would also appreciate an easily attached and removed test probe system adapted to snugly grip an IC device under test.
Others would similarly find useful a probe system including a removable attenuation clip and a repositionable grounding jumper to enable short-length connection of a measuring device cable to a plurality of extended pins of a device under test, at a predetermined impedance matching the input impedance of the measuring device.